The broad goal of this research is to explicate how the properties of sound map on the lexicon. The current proposal focuses on a characterization of the structure of phonetic categories, including both boundary constraints and internal structure, and the mapping of these categories on to lexical form. Boundary constraints of phonetic categories will be explored to determine whether particular acoustic properties called criterial attributes remain stable across a number of sources of variability including the influence of language, the presence or absence of phonemic contrast, the influence of the intrinsic nature of the sound segment (consonant or vowel), the influence of a secondary articulation on the emergence of an acoustic property, and the influence of regressive assimilation and vowel environment. The 'shape' of the internal structure of phonetic categories will be charted by exploring the extent to which languages have 'preferred' or prototypical region for the representation of a phonetic category; the relationship between a criterial attribute and the 'family' of values associated with it to define the best exemplar for that category; and the extent to which to which a range of 'best exemplars' of a criterial attribute may be influenced by speaking rate and the presence of other acoustic parameters. The mapping of phonetic categories on to lexical form will be investigated by assess whether and in which ways the acoustic variations which to and affect phonetic category structure ultimately impact lexical form. Experiments will investigate the role of acoustic variants that do or do not define phonetic categories, the influence of phonetic position and syllable stress, and the influence of prior lexical activation (top-down processing) on the emergence of subphonetic effects on lexical access. Research methods include acoustic analyses of speech and development of quantitative algorithms, computer-editing of natural speech in order to conduct both speech perception and lexical decision experiments. This research should provide a greater understanding of the speech processing system and how it interfaces with higher levels of language processing. Characterizing the speech processing of normal adults serves as the basis for the understanding of pathologies of speech and language, particularly in adults, but also in children.